Alloys with high resistance to sea water corrosion



United States Patent 3,268,327 ALLOYS WITH HIGH RESISTANCE TO SEA WATER CORROSION William O. Binder, Niagara Falls, N.Y., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Nov. 14, 1963, Ser. No. 323,550 9 Claims. (Cl. 75134) This is a continuation-in-part of application Serial No. 17,752, filed March 28, 1960, now abandoned.

The present invention relates to alloys with high resistance to sea water corrosion. In particular, the invention relates to alloys resistant to sea water corrosion and at the same time exhibiting good workability.

In the past, several alloys have been developed which are characterized by good sea water corrosion resistance especially with reference to freely flowing sea Water which is aerated. These alloys are limited generally to use in exposure to sea water which is freely flowing but they do not resist pitting corrosion nor do they exhibit good workability.

Pitting corrosion, as used herein, refers to accelerated attack in certain restricted areas of the surface of a metallic article caused by the formation of concentration cells resulting from surface irregularities and non-uniform aeration. These irregularities may be an integral part of the surface metal, such as crevices and pits themselves, or created by external agencies such as gaskets, packing and fasteners, fouling organisms, mill scale or high surfaceenergy concentrations due to stress caused by the physical integration of the constituent metals of the alloy surface itself.

It is an object of this invention to produce an alloy having a high resistance to sea water corrosion in general and especially to pitting corrosion.

Another object of this invention is to produce a corrosion-resistant alloy of good workability, thus making it easy to fabricate.

The objects of the present invention are accomplished by an alloy comprising 30 to 60 weight percent cobalt, 18 to 22 weight percent chromium, 3 to 7 weight percent molybdenum, 0 to weight percent nickel, 0 to 7 weight percent tungsten, O to 2 weight percent manganese, 0 to 1 weight percent silicon, O to 0.15 weight percent carbon and the remainder iron and incidental impurities, the amount of iron being at least 20 weight percent the sum of chromium and molybdenum being 25 weight percent.

The elements cobalt, chromium and molybdenum coact to provide superior resistance against sea water corrosion especially the phenomenon commonly termed pitting corrosion. The mechanical workability of the basic corrosion resisting alloy is enhanced by the addition of iron and nickel in certain prescribed amounts. Tungsten may be added in amounts up to 10 weight percent to impart added strength without detriment to corrosion resistance. Carbon, silicon and manganese are unavoidable tramp elements and must not be present in greater amounts than hereinbefore described. The tramp elements do not detract from the desirable properties of the final alloy if kept within the specified limits.

The alloys of the present invention are characterized by the fact that the desirable properties are obtained when the proportions of its constituents are maintained within specific limits. The desirable properties drop off sharply when the end points of the ranges are exceeded. As a result of careful experimentation, it has been discovered that adequate resistance to under-gasket corrosion can be achieved in an l822 weight percent chromium, 3-7 weight percent molybdenum alloy when cobalt is present in excess of 30 weight percent. The beneficial efiect of nickel on the workability of such alloys is well known,

but it has been further discovered that in order to retain adequate resistance to under-gasket corrosion, while at the same time provide good workability, nickel should be kept below 10 weight percent.

The ranges of composition of the alloys of this invention which produce the improved properties are 'set forth in the following table:

The first column indicates the maximum range of constituents which will produce a suitable alloy. The second column shows the range of constituents which will produce a high-grade alloy for general use, and the third column shows the composition which has been found most satisfactory in corrosion resistant properties in view of good workability.

To show the paramount importance of the above recited criticalities of the constituent elements, the alloys were exposed to a laboratory corrosion solution which was corroborated by tests under actual marine conditions. The medium used in these tests and found to give significant results was a solution containing 5% ferric chloride plus 10% sodium chloride. A gasket-type specimen was devised for evaluating resistance to corrosion perpetrated by inhibited flow. The specimen consisted of a sheet or plate about 3 to 4 inches long, 2 inches wide by convenient thicknesses, with one or two A-inch diameter holes present in order that gaskets might be brought into intimate contact with the surfaces. In order to avoid galvanic corrosion from dissimilar metals, Bakelite was used as the inert washer material to make contact with the specimen and form the crevice area, and also was used as the insulating material to prevent contact between the bolt and specimen.

TABLE II Influence of cobalt and nickel on alloys containing 20% chromium and 5% molybdenum Annealed by heating at temperatures of 1,050 O. to 1,200 C. and air-cooling or water-quenching.

2 Annealed and stress-relieved by heating 2 hours at 750 to 870 C. and air-cooling.

The above alloys contained less than 1.5 weight percent manganese, less than 0.5 Weight percent silicon and less than 0.05 weight percent carbon.

The induction-melted alloys were hot-rolled to sheet approximately /s-inch thick and, in order to determine the influence of heat-treatment, the specimens were tested in the solution-annealed and stress relieved conditions. The former renders the alloy most corrosion resistant While the latter may cause some loss in corrosion resistance.

Additional tests were run under actual sea-water over an extended period of approximately 6.5 years on applicants alloys in conjunction with a common marine alloy. The alloys were submitted to identical heat treatments prior to the initiation of the test so as to ascertain the effect of heat treatment on the alloys in addition to a compositional comparison.

Table III gives the results of the sea-water tests, one, identified as A is outside the range of the invention, and the other, identified as B is of the preferred composition. Panelspecimens 12 x 3 x fis-inch were prepared from hot-rolled material, solution-annealed by heating for 30 minutes at 1150 C. and air-cooled. In order to determine the influence of heat-treatments, such as those ordinarily encountered in fabrication, the panels were tested in the following conditions: as-solution-annealed; annealed followed by stress-relieving treatment for two hours at 870 C. Each specimen was insulated from the metallic holder bolts by Bakelite.

TABLE HI The above alloys contained less than 1.5 Weight percent manganese, less than 0.5 weight percent silicon and less than 0.05 weight percent carbon.

Tensile tests were made on the cobalt-bearing alloys A and B from using %-inch diameter specimens prepared from solution-annealed and a stress-relieved (2 hours at 870 C.) materials. The results of the tests, along with data for several other commonly known alloys which are noted sea-water corrosion resistance are noted in the following Table IV.

In Table IV, the following designations are used:

Alloy A is the preferred composition of applicants alloys with A denoting an alloy identical to Alloy A but which has been submitted to a different heat treatment;

Alloy B and B is a commonly used sea-water corrosion resistant alloy with E being of the same alloy composition as B but treated in a different manner; C is another prior art alloy commonly used in marine applications; type 316 Stainless Steel specimen is self explanatory;

All the specimens include minor amounts of manganese and silicon as tramp elements in the iron component. All the tensile specimens were 1.5 inch specimens except alloy C which was 2 inches in length;

Results of sea-water tests on the experimental data Approximate Composition, Maximum Depth of Gor- Percent rosion Under Gasket (Mils) Alloy Outside Heat-Treatment Invention After After After Co Ni Cr Mo Fe 5 mo. 18 1110. 6.4 Yr.

A 10 5 Bal..- min. 1,150 C. air-cooled 10 8 11 30 min. 1,150 O. air-cooled plus 2 hours 47 51 63 870 0., air-cooled.

\ Alloy Within Applicant's Invention B 5 20 5 Bal--- 30 min. 1,150 G. air-cooled 0 0 1 30 min. 1,150 O. air-cooled plus 2 hours 0 0 1 870 0., air-cooled. 30 min. 1,150 C. Heliarc Welded 0 0 1 30 min. 1,150" 0. air-cooled Heliarc welded 0 O 1 plus 2 hours 870 0., air-cooled.

It is obvious from the table that the preferred composition of the invention possesses superior resistance. It is to be further noted from the table that the alloys of this in Also all specimens in Table IV contain less than 1.5 weight percent manganese, less than 0.5 weight silicon and less than 0.05 weight percent carbon.

TABLE IV Tensile properties Approximate Composition, 7;, Yield Ultimate Percent; Percent Identifier Heat Treatment Strength Tensile Elong. Red. in

' 0.2% Offset, Strength, 1.5 inch area Co Ni 01' Mo Ta+Cb Fe p.s.i. p.s.i.

A 35 5 20 Bal- Solution-Annealed 51, 300 120, 900 60. 0 56. 6 A1 35 5 20 Bal Solution-Annealed plus 51, 800 120, 300 58. 6 51. 9

2 hr. at 870 C B 10 15 20 BaL Solution-Annealed 41, 800 98, 100 54. 6 63. 8 B1. 10 15 20 13211.. Solution-Annealed plus 40, 800 100, 000 63. 3 59. 7

2 hr. at 870 C.

C 45 22 6 2 Bal--- Solution-Annealed 45, 000 102, 700 46. 0 Type 316 Stainless Steel 12 18 2. 5 Bill-.- do 30, 000 75, 000 40. 0 50. 0

vention can be welded without fear of producing corrosion-sensitive areas which is so often the case with conventional alloys.

What is claimed is: 1. An alloy consisting essentially of 30 to weight percent cobalt; 19 to 21 weight percent chromium; 4 to 6 weight percent molybdenum; wherein the sum of chromium and molybdenum is 25 weight percent; 2 to 8 weight percent nickel; up to 7 weight percent tungsten; up to 2 weight percent manganese; up to 1 weight percent silicon; up to 0.15 weight percent carbon; and the remainder iron and impurities with iron being at least percent of the composition of the alloy.

2. An alloy consisting essentially of 35 Weight percent cobalt; 20 weight percent chromium; 5 weight percent molybdenum; 5 weight percent tungsten; up to 2 weight percent manganese; O to 1 weight percent silicon; up to 0.15 weight percent carbon; and the remainder iron and impurities with iron being at least 20 percent of the com position of the alloy.

3. An alloy consisting essentially of 50 weight percent cobalt; 20 weight percent chromium; 5 weight percent molybdenum and weight percent iron.

4. An alloy consisting essentially of 45 weight percent cobalt; 20 weight percent chromium; 5 weight percent molybdenum and percent iron.

5. An alloy consisting essentially of 40 weight percent cobalt; 20 weight percent chromium; 5 weight percent molybdenum and weight percent iron.

6. An alloy consisting essentially of 35 weight percent cobalt; 20 weight percent chromium; 5 weight percent molybdenum and weight percent iron.

References Cited by the Examiner UNITED STATES PATENTS Re.20,877 10/1938 Prange -171 2,247,643 7/1941 Rohn etal 75-171XR 2,381,459 8/1945 Merrick 75 171 2,432,619 12/1947 Franks et a1.

2,524,660 10/1950 H-arderet a1 75-171XR 2,704,250 3/1955 Pays-on 75-171 HYLAND BIZOT, Primary Examiner.

RICHARD DEAN, Examiner. 

1. AN ALLOY CONSISTING ESSENTIALLY OF 30 TO 40 WEIGHT PERCENT COBALT; 19 TO 21 WEIGHT PERCENT CHROMIUM; 4 TO 6 WEIGHT PERCENT MOLYBDENUM; WHEREIN THE SUM OF CHROMIUM AND MOLYBDENUM IS 25 WEIGHT PERCENT; 2 TO 8 WEIGHT PERCENT NICKEL; UP TO 7 WEIGHT PERCENT TUNGSTENS; UP TO 2 WEIGHT PERCENT MANGANESE; UP TO 1 WEIGHT PERCENT SILICON; UP TO 0.15 WEIGHT PERCENT CARBON; AND THE REMAINDER IRON AND IMPURITIES WITH IRON BEING AT LEAST 20 PERCENT OF THE COMPOSITION OF THE ALLOY. 